Under the circumstances that equal intensity point source is discharged off-bank in rivers of medium width with the consideration of the reflection of both banks, a calculation method of pollutant mixing zone was derived from the analytical solutions of the simplified two-dimensional advection-diffusion equation. The form of the boundary shape curves and the non-dimensional equation of the pollutant mixing zone were obtained. Using the methods of iterative calculation and trial calculation, pollutants mixing zones of river bank and off-bank were identified. The relationship between the geometric characteristic parameters(coordinates), the area and area coefficient of the zone, and the relative off-bank distance were found. As a result, the relative off-bank distance a′was proposed as a classification criterion of discharge types. The classification criteria for river bank, off-bank and river center discharge types are as follows: (1)for 0≤a′≤a₁′, the outfall can be simplified as the type of river bank(near-bank)discharge; (2)for a₁′<a′<a₂′, it is named as the type of off-bank discharge; (3)for a₂′≤a′≤0. 5, the outfall can be simplified as the type of river center discharge, and the expressions of a₁′and a₂′ have been put forward to represent the classification lines of the river bank and off-bank discharge, off-bank and river center discharge respectively. By regression analysis, an empirical formula of the main characteristic parameters for off-bank discharge in pollutant mixing zone in rivers of medium width is proposed, which provides the basis for the optimization of outfall locations and for the calculation of pollutant mixing zone.